1. Field of the Invention
The present invention relates to an optical fiber amplifier applied when the capacity and the distance of a WDM transmission system are increased.
2. Description of the Related Art
In recent years, the demand for increasing the amount of information has been sharply increasing with the progress of Internet technologies. Additionally, a further increase in a capacity and flexible network building are required for a trunk line optical transmission system on which the amount of information is concentrated.
In the present situation, a WDM (Wavelength Division Multiplexing) transmission method is the most effective means that meets such a system demand, and the U.S. is currently taking the initiative in commercializing the transmission system.
An amplifier on which a rare-earth element is doped to amplify a light, such as an EDFA (Erbium-Doped Fiber Amplifier), can collectively amplify a wavelength-multiplexed signal light by using its wide gain band. Therefore, this is a key component for implementing a WDM optical transmission system.
It is known that the EDFA can cover not only an amplification band (1530 to 1565 nm) called a conventional band (C-band), which is mainly used so far, but also an amplification band (1570 to 1605 nm) called a long wavelength band (L-band), the coverage of which has been enabled in recent years.
The current EDFA system allows a wavelength-multiplexed signal including signal lights of approximately 200 wavelengths to be amplified in a band into which the C-band and the L-band are combined.
With the EDFA, an inversion population ratio must be selected so that the gain of each wavelength-multiplexed signal light becomes equal in a wavelength band to be used.
FIG. 1 shows the wavelength dependency of the gain coefficient per unit length of an EDF (Erbium-Doped Fiber) when the inversion population ratio is varied.
Namely, the graph of this figure shows the average of the inversion population ratio of the EDF over a predetermined length in the longitudinal direction of the EDF. The characteristic with the smallest gain shown in FIG. 1 indicates the state where a rare-earth element is not pumped (inversion population ratio is 0), whereas the characteristic with the largest gain indicates the state where all the atoms of a rare-earth element are excited (inversion population ratio is 1). Characteristics between them respectively indicate the case where the inversion population ratio is incremented by 0.1.
The following points are known from FIG. 1.
(1) The C-band EDFA is in the neighborhood of the central wavelength (1530 nm) of the emission/absorption of an Er ion. A sufficient gain can be secured with a short EDF.
(2) The L-band EDFA per unit length is small. A longer EDF is required in comparison with the C-band EDFA.
FIG. 2 shows the fundamental configuration of an L-band EDFA for optical amplification.
1 indicates an EDF, 2-1 and 2xe2x80x942 indicate optical isolators, 3 indicates a wavelength-division multiplexing (WDM) coupler, 4 indicates a semiconductor laser (LD) for pumping, and 5 indicates a transmission line.
A wavelength-division multiplexed signal light supplied from the transmission line 5 is provided to the EDF 1 via the optical isolator 2-1 and the WDM coupler 3. The EDF 1 must be made long so as to provide an L-band signal light with almost the same gain, and to make the gain equal to that of a C-band signal light.
In an optical amplifier of a gain shift type, whose inversion population ratio to obtain a flat gain for a target amplification wavelength band is low, and whose rare-earth element doped fiber length must be made long to obtain a necessary gain, such as an L-band EDFA, amplification must be performed stably and efficiently.
Several results of studying the above described optical amplifier of a gain shift type, such as an L-band EDFA, are described below.
As shown in FIG. 1, the length of an L-band EDF must be made longer than that of a C-band EDF so as to obtain a gain equal to that of the C-band in the L-band.
FIG. 3 shows the inversion population ratio in the longitudinal direction resultant from a simulation where an EDF is forward pumped with a constant pump light source power.
The following two points are known from FIG. 3.
(1) The inversion population ratio of the Er ion of the EDF length required for an L-band EDFA is low in the neighborhood of an output end. This is because the EDF length is long, and the pump light power does not fully reach the output end.
(2) The inversion population ratio of the Er ion of the EDF length required for a C-band EDFA is high throughout the EDF. This is because the EDF length is short, and the pump light source power fully reaches also the output end.
FIG. 4 shows the outputs of respective wavelengths in the longitudinal direction of an EDF, which are resultant from the simulation where signal lights having the same power are positioned at intervals of approximately 0.4 nm, and the 80 wavelengths are multiplexed between 1570 to 1605 nm (within the L-band)
In this figure, the highest characteristic indicates a 1570-nm signal light channel ch1, from which signal channels start to be increased in units of 8 downward, and the lowest characteristic indicates a 1605-nm signal light channel ch 80.
It is known from FIG. 4 that there are significant differences among the respective signal light channels in the power distribution in the longitudinal direction.
Namely, for the L-band EDFA, there are a tendency such that lights on the short wavelength side secure a large gain in the neighborhood of the input end, and decrease in power toward the output end after reaching a maximum value, and a tendency such that lights on the long wavelength side monotonously increase in power from the input end toward the output end.
Namely, it is known from FIGS. 3 and 4 that the wavelength characteristic of a gain significantly changes as the inversion population ratio varies.
Accordingly, a long EDF required for the L-band EDFA is proved to have an effect that a signal light on the long wavelength side in the neighborhood of an output end contributes to optical amplification with energy that absorbs signal light power on the short wavelength side, as is known from the above described tendency of FIG. 3 such that the inversion population ratio decreases in the neighborhood of the output end of the long EDF used for the L-band EDFA, and the output value and the wavelength characteristics in the longitudinal direction in FIG. 4.
Assume that signal lights having arbitrary wavelengths among the 80 signal light channels shown in FIG. 4 are amplified unchanged as being wavelength-multiplexed in the system configuration shown in FIG. 2. Further assume that lights of 2 wavelengths such as a signal channel ch1 on the shortest wavelength side and a signal light channel chX on its longer wavelength side are input to the optical fiber amplifier, and collectively amplified on the condition that the pump light power is constant. In this case, if a comparison is made between the output of the signal light channel chX on the longer wavelength side in the case where the channels are collectively amplified, and the output of the signal light channel chX in the case where the signal light channel ch1 on the shortest wavelength side is dropped, a phenomenon that the power of the signal light channel chX on the longer wavelength side in the case where the signal light channel ch1 on the shortest wavelength side is dropped is sometimes lower, occurs.
FIG. 5 assumes the system having the configuration of FIG. 2 in which an arbitrary number of wavelengths among the 80 channels shown in FIG. 4 are multiplexed, and shows the dependency of the varying output of the signal light channel ch80 on the EDF length in the case where the signal light channel ch1 is driven on the condition that the pump light power is constant, and in the case where the signal light channel ch1 is dropped.
With a predetermined EDF length or longer, the output level of the signal light channel ch80 becomes lower. With the EDF length required for optically amplifying an L-band light, the output level of the signal light channel ch80 becomes lower when the signal light channel ch1 on the shortest wavelength side is dropped under the above described condition.
In the meantime, with the EDF length required for a C-band light, the output level of the signal light channel ch80 becomes not lower but higher.
The above described phenomenon can be explained by the relationship of the energy level of the Er ion within the EDF, and by FIGS. 1 and 3.
FIG. 6 shows the energy level of the Er ion within the EDF for an L-band EDFA.
In this figure, a state transition is made from 4I15/2 to 4I11/2 by 0.98-xcexcm pumping, a state transition is made from 4I15/2 to 4I13/2 by 1.48-xcexcm pumping, a state transition is made from 4I13/2 to 4I15/2 by spontaneous emission occurring in a 1.55- to 1.57-xcexcm band, a state transition is made from 4I13/2 to 4I15/2 by GSA (Ground State Absorption), and a state transition is made from 4I13/2 to 4I15/2 by induced emission with an induced light (an L-band signal light) in a 1.55- to 1.61-xcexcm band.
Viewing the characteristics shown in FIG. 1, the gain coefficient is smaller than 0 in the L-band if the inversion population ratio is lower than 0.3, so that an absorption coefficient is larger than the gain coefficient, and the GSA shown in FIG. 6 exceeds the radiation transition (spontaneous emission, and induced emission). Additionally, the absorption coefficient tends to be larger on the shorter wavelength side.
Furthermore, in FIG. 3, the inversion population ratio is lower than 0.3 at the length exceeding approximately 20 m. Accordingly, a signal light on the long wavelength side is considered to be amplified with the GSA of a signal light on the short wavelength side in the neighborhood of the output end, which exceeds approximately 20 m, in the L-band EDFA.
Normally, the GSA of a signal light wavelength is dominant not in the amplification of a C-band light of a short EDF, but in the neighborhood of the output end of a long EDF for the amplification of an L-band light. Therefore, absorption on the short wavelength side becomes larger as described above. Accordingly, the presence/absence of a channel on the short wavelength side exerts more influence on the amplification of an L-band light.
FIG. 7 assumes the case where lights of arbitrary wavelengths are multiplexed among the 80 signal light channels shown in FIG. 4, and shows a specific example of the power distribution of the channel ch1 on the shortest wavelength side and the channel ch80 on the long wavelength side in the longitudinal direction of an EDF.
In this figure, a pump light input to the EDF is assumed to be a forward pump for a signal light, and the power of the pump is made constant. xe2x80x9caxe2x80x9d indicates the output characteristic of the signal light channel ch1 when the signal light channels ch1 and ch80 are optically amplified. xe2x80x9cbxe2x80x9d indicates the output characteristic of the signal light channel ch80 when the signal light channels ch1 and ch80 are optically amplified. xe2x80x9ccxe2x80x9d indicates the output characteristic of the signal light channel ch80 when only the signal light channel ch80 is optically amplified.
As shown in FIG. 7, a phenomenon that the signal light channel ch80 is significantly attenuated at the output end at the time of the amplification of only the signal light channel ch80, but the gain of the ch80 at the time of the amplification of the two wavelengths is larger, even with the same pump light power, than the gain at the time of the amplification of only one wavelength, occurs.
Even if feedback control works on the pump light, for example, to make the gain or the output level constant, the output of the signal light channel ch80 becomes transiently low, and an error can possibly occur instantaneously in the signal light channel ch80 at a receiving end due to the above described phenomenon, if the signal light channel ch1 is blocked by any trouble, etc. when the signal optical channels ch1 and ch80 are used. Furthermore, this phenomenon is considered to occur not only when an Er-doped fiber optical amplifier for an L-band is used, but also when a relatively long fiber, such as a Tm-doped fluoride fiber optical amplifier made by decreasing the inversion population coefficient, and by making the fiber length longer to obtain a gain (a gain shift type similar to an EDFA), is used.
It is an object of the present invention to provide an optical fiber amplifier that does not deteriorate an amplification characteristic on a long wavelength side even if a signal on a short wavelength side is disconnected in a wavelength-multiplexing amplifier.
An optical fiber amplifier according to the present invention comprises: an amplifying unit, which is doped with a rare-earth element, amplifying a signal light; a pump light source unit supplying optical energy for amplifying the signal light; and an ASE returning unit returning a light having a wavelength that is longer than a wavelength with which an ASE light occurs with a maximum efficiency, and shorter than an amplification band among ASE lights occurring when the signal light is amplified.
According to the present invention, optical energy is supplied to the amplifying unit by returning an ASE light to the amplifying unit, and used to amplify a signal light on the long wavelength side of a wavelength band to be amplified. As a result, an optical fiber amplifier having a stable amplification characteristic can be provided without deteriorating the amplification gain of the signal light having a long wavelength, even if a signal light having a short wavelength within the amplification band is instantaneously disconnected.